The invention relates generally to machine condition monitoring. More particularly, the invention relates to a dynamic triggering hold off for a phase sensor for a moving element of a machine.
In the field of machinery diagnostics and protection, a sensor signal that has an output signal related to the machine speed is required. Typically, the sensor signal is a pulse signal generated once-per-event from a projecting key or a notch on a moving element of the machine such as a rotating shaft. In the case of a rotating shaft, a notch passing the sensor once-per-turn triggers the sensor. The once-per-turn event is useful not only for determining the shaft speed but also to provide a phase reference for comparing against other shaft sensor signals. For example, the phase difference between the phase reference and a radial vibration transducer signal provides an indicator of machine condition.
Circuitry determines when the sensor triggers or, in other words, creates a signal indicative of the tested event, such as a notch on the rotating shaft passing the sensor. As a speed signal and phase reference, it is critical that the circuit that controls triggering of the sensor triggers only once per event. However, ensuring once per event triggering can be difficult due to noise sources such as electrical runout, mechanical runout, inadequate machining of the notch or projection, and overall machine vibration.
FIGS. 1 and 2 show examples of the problem in terms of a rotating shaft. The upper signal (series 1) in each figure is a raw input signal, and the lower signal (series 2) in each figure is the triggered phase signal from a speed or phase sensor. The downward spikes in the raw input signal indicate where the sensor is triggered. As illustrated, there is a large sinusoidal vibration occurring synchronous to the machine speed. The desired triggering in the phase signal is indicated by the downward pulse(s) 10, and is supposed to occur each time the sensor passes over a notch in the rotating shaft. As indicated, there is a significant amount of noise on the raw input signal (indicated by fuzziness of data) which may be caused by, for example, electrical runout on the shaft itself or from other electrical interference. The triggered phase pulses are not occurring at the same frequency as the vibration signal. Note that the triggering problem becomes worse as the phase between the vibration and notch becomes such that the notch occurs at the peak of the vibration. In FIG. 1, the signal is triggering off both the desired event and the vibration noise signal. That is, the noise is creating false triggering (e.g., pulse(es) 12) in the phase signal in FIG. 1. In FIGS. 1 and 2, the phase signal is triggering multiple times from the noise, as indicated by the clumped, multiple downward pulses 14 in the phase signal. Commonly, a triggering error results in an extra pulse as a noise event passes across the triggering level. The trigger level is a signal amplitude of the raw input signal sufficient to indicate an event occurrence.
In previous machine condition monitoring, protection and diagnostics systems, phase pulses have been processed using a trigger level and hysteresis. That is, a system triggers the output phase pulse when the signal crosses the trigger level minus a hysteresis in a negative direction, and then accepts a new trigger only after the raw input signal crosses the threshold plus a hysteresis in the positive direction. While this method works well for signals where the signal-to-noise ratio is good, when the noise is high and the notch or projection is shallow or otherwise defective, it can be difficult to choose an appropriate hysteresis and trigger level to prevent unwanted retriggering. Oscilloscopes have historically provided a hold off time that must pass before a signal will be allowed to re-trigger. However, this hold off time is stagnant and is not calculated or optimized for use on machines having moving elements such as a rotating shaft.